The invention relates to a power device, and in particular to a power device applied in a display device.
FIG. 1 is a schematic diagram of a panel of a conventional organic light emitting display (OLED) device. A panel 1 comprises a data driver 10, a scan driver 11, and a display array 12. The data driver 10 controls a plurality of data lines D1 to Dn, and the scan driver 11 controls a plurality of scan lines S1 to Sm. The display array 12 is formed by the interlaced data lines D1 to Dn and scan lines S1 to Sm. Each interlaced data line and scan line control a display unit of the display array 12. For example, the data line D1 and the scan line S1 control a display unit 100. As with any other display unit, the equivalent circuit of the display unit 100 comprises a switch transistor T10, a storage capacitor C10, a driving transistor T11, and a light-emitting diode (LED) D10.
An OLED device is a self-illuminating flat panel display. Light from the LED D10 is transformed from current I flowed itself. The brightness of the LED D10 can be determined according to the current I provided by the driving transistor T11. In the panel 1, voltage Vdd, provided to the driving transistor T11, must be adjusted due to process derivation of the driving transistor T11, thus, the brightness from all the display units of the display array 12 reaches a predetermined level. In general, the adjustment range of the voltage Vdd is 2V.
Referring to FIG. 2, an external power device provides the voltage Vdd and Vss to each display unit. The external power device provides the power consumed by all LEDs of the display array 12. In the display array 12, nodes of the voltage Vdd are connected, and nodes of the voltage Vss are connected. The nodes of the voltage Vdd and Vss are led to outer edges of the panel 1 and connected to the external power device 2 through leads.
Conventional external power devices adjust voltage Vdd and Vss or voltage Vdd only. FIG. 3a shows a conventional external power device adjusting voltage Vdd. An external power device 2 comprises DC/DC converters 31 and 32 and adjusting devices 33 and 34. The DC/DC converters 31 and 32 respectively provide voltage Vdd and Vss. Both adjusting devices 33 and 34 have two impedance elements R. A value of an impedance element R1 of the adjusting devices 33 is adjustable, and a value of an impedance element R2 thereof is fixed. When the DC/DC converter 31 provides the voltage Vdd to the panel 1, a feedback voltage Vf1 is acquired by dividing the voltage Vdd by the impedance elements R1 and R2. The DC/DC converter 31 determines the value of the voltage Vdd according to the feedback voltage Vf1. When the voltage Vdd requires adjustment due to the process derivation of the transistor T11, the feedback voltage Vf1 is varied by adjusting the value of the impedance elements R1. The DC/DC converters 31 thus adjust the value of the voltage Vdd according to the varied feedback voltage Vf1. Since the DC/DC converter 32 provides the fixed voltage Vss, the values of the impedance elements R3 and R4 are fixed. For a panel only requiring 10V cross-voltage, which is defined by the voltage between Vdd and Vss, the cross-voltage of the entire panel varies between 10V and 12V, resulting in a maximum consumption increment of 20% power for the panel.
FIG. 3b shows a conventional external power device adjusting both voltage Vdd and Vss. An external power device 3 comprises DC/DC converters 35 and 36 and adjusting devices 37 and 38. The DC/DC converters 35 and 36 respectively provide voltage Vdd and Vss. Both adjusting devices 37 and 38 have two impedance elements R. A value of an impedance element R5 of the adjusting devices 37 is adjustable, and a value of an impedance element R6 thereof is fixed. When the voltage Vdd needs to be adjusted due to process derivation of the transistor T11, the feedback voltage Vf1 is varied by adjusting the value of the impedance elements R5, and the DC/DC converters 35 thus adjusts the value of the voltage Vdd according to the varied feedback voltage Vf1. After the voltage Vdd is adjusted, the voltage Vss is adjusted to avoid the excess power consumption, so that the cross-voltage between Vdd and Vss is maintained at 10V. In the external power device 3, a value of an impedance element R7 of the adjusting devices 38 is adjustable, and a value of an impedance element R8 thereof is fixed. The DC/DC converter 36 and adjusting devices 38 perform the same operation respectively as the DC/DC converter 35 and adjusting devices 37. When the voltage Vss needs to be adjusted due to the process derivation, the feedback voltage Vf2 is varied by adjusting the value of the impedance elements R7, and the DC/DC converters 36 thus adjusts the value of the voltage Vss according to the varied feedback voltage Vf2. According to the external power device of FIG. 3b, when process derivation of the transistor T11 occurs, not only the voltage Vdd but also the voltage Vss is adjusted, resulting in an increased number of manufacturing processes for OLED devices.